Wavelength division multiplexing (WDM) or dense WDM (DWDM) communication systems increase the transmission capacity of an optical fiber in a communication system by combining several optical signals with different wavelengths. To render these systems practical, optical amplifiers must meet tight requirements for providing gain uniformity over all channel signal wavelengths. Since the intrinsic gain spectrum of a rare earth doped fiber amplifier is highly non-uniform, some method of flattening the gain profile of the amplifier must be implemented. The gain profile of the fiber amplifier, however, is complicated by other factors as well. The gain profile of a rare earth doped optical fiber amplifier OFA), such as an erbium doped fiber amplifier (EDFA), is determined by the average inversion level of the erbium ions in the erbium doped fiber. This inversion level is a function of the power level of the signal or signals to be amplified and the applied power levels of the pump sources. If the signal power is sufficiently lower than the applied pump power, the fiber will maintain close to 100% inversion and the signal gain and the amplifier gain profile will not appreciably change with changes in input signal powers. However, as signal powers increase, the signal gain in the amplifier becomes limited by the availability of pump power for the fiber amplifier, i.e., output signal power is, of course, limited by the available pump power and the signal input power levels. OFA's usually operate with their gain fibers saturated because this provides for optimum pump to signal power conversion. When the gain of the amplifier is "saturated", the inversion level, amplifier gain, and the gain profile of the amplifier all become strongly dependent on the ratio of signal to pump powers. The standard manner of maintaining constant gain flatness with changes in input power levels is to actively change the pump power to all OFA stages of the system in order to maintain constant signal to pump power ratios. There is a need, therefore, to provide a WDM amplifier system with flexibility to provide a commercially viable way to provide for maintenance of constant pump to signal ratios without redesigning the amplifier system. It is good to accomplish this by increasing the pump power but this must be done without saturating the input amplifier stage of the system, also referred to as the pre-amplifier; otherwise, the noise figure is materially affected.
While much progress has been made in achieving wide bandwidth gain flattened OFA systems, such as the system illustrated in Masuda et al., "Wideband, Gain-Flattened, Erbium-Doped Fibre Amplifier With 3 dB Bandwidths of &gt;50 nm", Electronic Letters. Vol. 33(12), pp. 1070-1072, Jun. 5, 1997, the achievement of higher output powers required to make these systems more practical have not achieved much attention as well as optimization and adjustability of the gain flattening characteristics of the system.
In providing additional pump power, it is desired that the OFA system lend itself to be easily adaptability to power upgradability, allowing customers to purchase a lower capacity amplifier system and, later, upgrade the system to increase system capacity with increasing communication demands while providing the same output gain per channel and no significant change in system noise figure.
Therefore, it is an object of this invention to provide a multistage fiber amplifier system that is field upgradable in pump power and channel capacity.
It is another object of this invention to provide a multistage fiber amplifier system that is upgradable to higher output power levels and increase in channel capacity while maintaining uniform signal output per channel.
A further object of this invention to provide an upgradable multistage fiber amplifier system that allows a wide variation in possible external gain configurations while employing the same interstage gain equalizer continually maintaining uniform amplifier gain flattening in spite of changes to the pump power and signal channel capacity to the base multistage amplifier system.